This invention relates to communications systems in general, and more particularly to an independent single sideband (SBB) transmission and reception apparatus.
The prior art is replete in describing and specifying various types of communications systems employing multiplexing, various modulation schemes or combinations of such techniques to achieve maximum bandwidth efficiency. As is known, various constraints in the bandwidth of a transmitted signal are determined by the particular broadcast band as well as by Governmental Agency regulations. Of particular concern in many communications systems is the transmission of voice at good intelligibility which occurs within a bandwidth of 300Hz to 3,000Hz.
A major industry involved in the transmission of voice and music is the AM or amplitude modulated broadcasting systems and those receivers employed by the public to receive such transmissions. Essentially, the AM broadcast bands covers the range of 550KHz to 1,600KHz, and various stations transmit within the band throughout the world. An AM transmission employs a transmitting bandwidth of about two hundred percent relative to the signal source bandwidth.
In any event, the extreme bandwidth employed is of little consequence as detection of the AM signal is relatively simple and a suitable receiver tuned to the AM band is a relatively reliable and economical apparatus.
The FM (frequency modulated) broadcasting stations have been increasing in activity and the sales of FM receivers are also increasing as compared to AM. The FM broadcast and bandwidth enables "stereo" transmission which provides the listener with an added dimension and hence, is preferred to AM in general.
In order to preserve AM and to provide the listener with stereo broadcast, a system generally designated as "SIMULCAST" was devised. In this system, a portion of the stereo data is transmitted on the AM carrier and the remaining information is transmitted on the FM carrier. In employing this method, a user must possess both an AM and FM receiver, and both the AM and FM transmitters broadcast the desired information simultaneously. Thus, the AM station still depends upon the FM station for implementation of the "SIMULCAST" transmission.
Generally speaking, a desireable apparatus would provide the AM broadcaster with the ability to transmit stereo information within the alloted bandwidth and to do so without substantially increasing the cost and simplicity of the receiving apparatus.
A search regarding the general nature of such systems was conducted in Class 325, sub-classes 45,47 and 141. While many patents were found, they are not deemed to be anticipatory of the techniques to be described herein and are as follows:
______________________________________ PAT. NO. DATE INVENTOR CLASS ______________________________________ U.S. 3,492,579 1/1970 Carassa 325/47 U.S. 3,579,111 5/1971 Johannessen et al 325/141 U.S. 3,603,882 9/1971 Wilson 325/47 U.S. 3,667,047 5/1972 Iwasaki et al 325/45 U.S. 3,757,220 9/1973 Abel 325/47 U.S. 3,824,470 7/1974 Eastmond 325/45 U.S. 3,909,722 9/1975 Bennett 325/45 U.S. 3,939,407 2/1976 Bickford 325/47 U.S. 3,962,638 6/1976 Sallis 325/45 ______________________________________